Add lesson 3

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diff --git a/Lesson_3/README.md b/Lesson_3/README.md
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+Finally we are ready to write our "Hello World" app.
+
+First we create a new edk2 module in our package directory similar to the our `SimplestApp` module:
+```
+$ mkdir UefiLessonsPkg/HelloWorld
+$ vi UefiLessonsPkg/HelloWorld/HelloWorld.inf
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = HelloWorld
+  FILE_GUID                      = 2e55fa38-f148-42d3-af90-1be247323e30
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = UefiMain
+
+[Sources]
+  HelloWorld.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+```
+Don't forget to add our newly created app to the `Components` section of the package DSC file
+```
+[Components]
+  UefiLessonsPkg/SimplestApp/SimplestApp.inf
++ UefiLessonsPkg/HelloWorld/HelloWorld.inf
+```
+Next we need to write the source code file. Let's remember the code for our SimplestApp:
+```
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  return EFI_SUCCESS;
+}
+```
+To print something to the console ("Hello World" message in our case) we need to use services from the `EFI_SYSTEM_TABLE` that is passed to the entry point of our app.
+
+The description of the `EFI_SYSTEM_TABLE` can be found in the UEFI specification (https://uefi.org/sites/default/files/resources/UEFI_Spec_2_8_final.pdf).
+
+EFI_SYSTEM_TABLE is a struct that was populated by the UEFI firmware and contains pointers to the runtime and boot services tables.
+```
+typedef struct {
+ EFI_TABLE_HEADER Hdr;
+ CHAR16 *FirmwareVendor;
+ UINT32 FirmwareRevision;
+ EFI_HANDLE ConsoleInHandle;
+ EFI_SIMPLE_TEXT_INPUT_PROTOCOL *ConIn;
+ EFI_HANDLE ConsoleOutHandle;
+ EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *ConOut;
+ EFI_HANDLE StandardErrorHandle;
+ EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *StdErr;
+ EFI_RUNTIME_SERVICES *RuntimeServices;
+ EFI_BOOT_SERVICES *BootServices;
+ UINTN NumberOfTableEntries;
+ EFI_CONFIGURATION_TABLE *ConfigurationTable;
+} EFI_SYSTEM_TABLE;
+```
+We are interested in the `ConOut` field. `ConOut` is abbreviaton for "Console Output" and according to the UEFI spec it is a pointer to the `EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL` interface that is associated with `ConsoleOutHandle`.
+
+If we keep digging into UEFI spec we can find description of the `EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL`.
+According to the spec `EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL` defines the minimum requirements for a text-based ConsoleOut device.
+
+As everything in UEFI it has GUID:
+```
+#define EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL_GUID \
+ {0x387477c2,0x69c7,0x11d2,\
+ {0x8e,0x39,0x00,0xa0,0xc9,0x69,0x72,0x3b}}
+```
+
+And the interface description is:
+```
+typedef struct _EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL {
+ EFI_TEXT_RESET Reset;
+ EFI_TEXT_STRING OutputString;
+ EFI_TEXT_TEST_STRING TestString;
+ EFI_TEXT_QUERY_MODE QueryMode;
+ EFI_TEXT_SET_MODE SetMode;
+ EFI_TEXT_SET_ATTRIBUTE SetAttribute;
+ EFI_TEXT_CLEAR_SCREEN ClearScreen;
+ EFI_TEXT_SET_CURSOR_POSITION SetCursorPosition;
+ EFI_TEXT_ENABLE_CURSOR EnableCursor;
+ SIMPLE_TEXT_OUTPUT_MODE *Mode;
+} EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL;
+```
+Right now we are interested in a `OutputString` method:
+```
+OutputString     Displays the string on the device at the current cursor location.
+```
+This is what we need. Let's look at the function description:
+```
+EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL.OutputString()
+
+Summary
+Writes a string to the output device.
+
+Prototype
+typedef
+EFI_STATUS
+(EFIAPI *EFI_TEXT_STRING) (
+ IN EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL *This,
+ IN CHAR16 *String
+ );
+
+Parameters
+This    A pointer to the EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL instance.
+String  The Null-terminated string to be displayed on the output device(s).
+```
+
+In edk2 `EFI_SIMPLE_TEXT_OUTPUT_PROTOCOL` is defined in the header file:
+https://github.com/tianocore/edk2/blob/master/MdePkg/Include/Protocol/SimpleTextOut.h
+
+
+With all this knowledge we can write our source code file `UefiLessonsPkg/HelloWorld/HelloWorld.c`:
+```
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  SystemTable->ConOut->OutputString(SystemTable->ConOut, L"Hello World!\n");
+  return EFI_SUCCESS;
+}
+```
+
+The `L""` signifies that the string is composed from CHAR16 symbols, as was required in spec.
+
+Let's compile our edk2 module:
+```
+$ build --platform=UefiLessonsPkg/UefiLessonsPkg.dsc \
+        --module=UefiLessonsPkg/HelloWorld/HelloWorld.inf \
+        --arch=X64 \
+        --buildtarget=RELEASE --tagname=GCC5
+```
+
+Copy the app to our `UEFI_disk` folder and run OVMF:
+```
+$ cp Build/UefiLessonsPkg/RELEASE_GCC5/X64/HelloWorld.efi ~/UEFI_disk/
+$ qemu-system-x86_64 -drive if=pflash,format=raw,file=Build/OvmfX64/RELEASE_GCC5/FV/OVMF.fd \
+                     -drive format=raw,file=fat:rw:~/UEFI_disk \
+                     -nographic \
+                     -net none
+```
+
+```
+UEFI Interactive Shell v2.2
+EDK II
+UEFI v2.70 (EDK II, 0x00010000)
+Mapping table
+      FS0: Alias(s):HD0a1:;BLK1:
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)/HD(1,MBR,0xBE1AFDFA,0x3F,0xFBFC1)
+     BLK0: Alias(s):
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+     BLK2: Alias(s):
+          PciRoot(0x0)/Pci(0x1,0x1)/Ata(0x0)
+Press ESC in 4 seconds to skip startup.nsh or any other key to continue.
+Shell> fs0:
+FS0:\> HelloWorld.efi
+Hello World!
+FS0:\>
+```
diff --git a/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.c b/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.c
new file mode 100644
index 0000000..399b0ac
--- /dev/null
+++ b/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.c
@@ -0,0 +1,10 @@
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  SystemTable->ConOut->OutputString(SystemTable->ConOut, L"Hello World!\n");
+  return EFI_SUCCESS;
+}
diff --git a/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.inf b/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.inf
new file mode 100644
index 0000000..95924c5
--- /dev/null
+++ b/Lesson_3/UefiLessonsPkg/HelloWorld/HelloWorld.inf
@@ -0,0 +1,17 @@
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = HelloWorld
+  FILE_GUID                      = 2e55fa38-f148-42d3-af90-1be247323e30
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = UefiMain
+
+[Sources]
+  HelloWorld.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
+
diff --git a/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.c b/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.c
new file mode 100644
index 0000000..8bdf500
--- /dev/null
+++ b/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.c
@@ -0,0 +1,10 @@
+EFI_STATUS
+EFIAPI
+UefiMain (
+  IN EFI_HANDLE        ImageHandle,
+  IN EFI_SYSTEM_TABLE  *SystemTable
+  )
+{
+  return EFI_SUCCESS;
+}
+
diff --git a/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.inf b/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.inf
new file mode 100644
index 0000000..7d4bae2
--- /dev/null
+++ b/Lesson_3/UefiLessonsPkg/SimplestApp/SimplestApp.inf
@@ -0,0 +1,16 @@
+[Defines]
+  INF_VERSION                    = 1.25
+  BASE_NAME                      = SimplestApp
+  FILE_GUID                      = 4a298956-fbe0-47fb-ae3a-2d5a0a959a26
+  MODULE_TYPE                    = UEFI_APPLICATION
+  VERSION_STRING                 = 1.0
+  ENTRY_POINT                    = UefiMain
+
+[Sources]
+  SimplestApp.c
+
+[Packages]
+  MdePkg/MdePkg.dec
+
+[LibraryClasses]
+  UefiApplicationEntryPoint
diff --git a/Lesson_3/UefiLessonsPkg/UefiLessonsPkg.dsc b/Lesson_3/UefiLessonsPkg/UefiLessonsPkg.dsc
new file mode 100644
index 0000000..91a79e6
--- /dev/null
+++ b/Lesson_3/UefiLessonsPkg/UefiLessonsPkg.dsc
@@ -0,0 +1,26 @@
+[Defines]
+  DSC_SPECIFICATION              = 0x0001001C
+  PLATFORM_GUID                  = 3db7270f-ffac-4139-90a4-0ae68f3f8167
+  PLATFORM_VERSION               = 0.01
+  PLATFORM_NAME                  = UefiLessonsPkg
+  SKUID_IDENTIFIER               = DEFAULT
+  SUPPORTED_ARCHITECTURES        = X64
+  BUILD_TARGETS                  = RELEASE
+
+
+[LibraryClasses]
+  UefiApplicationEntryPoint|MdePkg/Library/UefiApplicationEntryPoint/UefiApplicationEntryPoint.inf
+  UefiBootServicesTableLib|MdePkg/Library/UefiBootServicesTableLib/UefiBootServicesTableLib.inf
+  DebugLib|MdePkg/Library/UefiDebugLibConOut/UefiDebugLibConOut.inf
+  BaseLib|MdePkg/Library/BaseLib/BaseLib.inf
+  PcdLib|MdePkg/Library/BasePcdLibNull/BasePcdLibNull.inf
+  BaseMemoryLib|MdePkg/Library/BaseMemoryLib/BaseMemoryLib.inf
+  RegisterFilterLib|MdePkg/Library/RegisterFilterLibNull/RegisterFilterLibNull.inf
+  PrintLib|MdePkg/Library/BasePrintLib/BasePrintLib.inf
+  DebugPrintErrorLevelLib|MdePkg/Library/BaseDebugPrintErrorLevelLib/BaseDebugPrintErrorLevelLib.inf
+
+[Components]
+  UefiLessonsPkg/SimplestApp/SimplestApp.inf
+  UefiLessonsPkg/HelloWorld/HelloWorld.inf
+
+